1. Drawing Lewis Structures of Molecules Chapter 4 Section 4

2. Step 1 Use chemical symbols for the various elements to draw the skeleton structure of the compound. –Place least electronegative atom in the center. –Hydrogen, fluorine, and other halogens occupy end positions. –Carbon forms chains.

3. Step 2 Determine the number of valence electrons associated with each atom and combine them to determine the total number of valence electrons. –Example 1: F 2 2 F atoms x 7 electrons = 14 –Example 2: CH 3 Cl 1 C atom x 4 e - = 4 e - 3 H atoms x 1 e - = 3 e - 1 Cl atom x 7 e - = 7 e - 14 e -

4. Step 2 (cont.) If the molecule has a charge, add or subtract electrons accordingly. (add electrons for a - charge, subtract them for a + charge) SO 4 -2 (sulfate) NH 4 +1 (ammonium) 1 S = 6 e - 1 N = 5 e 4 O = 24 e - 4 H = 4 e charge = +2 e - charge = -1 e 32 e - 8 e

5. Step 3 Connect the central atom to each of the surrounding atoms using electron pairs. –All atoms want 8 electrons except H which wants 2. –Add electron pairs around the end atoms first. If any are left over, then put them around the central atom. –Check to make sure that the number of electrons drawn matches the number of total valence electrons calculated in step 2.

6. Step 4 Check the OCTET RULE ! If the octet rule is not satisfied for any atom, move one or more electron pairs from the surrounding atoms to create double or triple bonds. Halogens NEVER multiple bond.

7. Step 5 After you are satisfied with the Lewis structure you have constructed, perform a final electron count verifying the total number of electrons AND that they all have 8 (or 2 if it’s hydrogen).

8. Example: CH 4 Step 1: Draw a skeleton structure. H is never the central atom. H H C H H

9. Example: CH 4 Step 2: determine the number of TOTAL valence electrons H1 C = 4 v.e.’s H C H4 H = 4 v.e.’s H 8 total v.e.’s

10. Example: CH 4 Step 3: Connect central atom with surrounding atoms by electron pairs Make sure the number of electrons drawn equals the total number of electrons calculated in step #2

11. Example: CH 4 Step 4: Check the OCTET RULE Do all Hydrogens have 2 electrons and all other atoms have 8?

12. Example: CH 4 Step 5: Verify that the total number of electrons is correct 8 total electrons

13. Now, with a partner, try these! NH 3 CF 4 CH 3 Cl C 3 H 8

14. HOMEWORK Draw Lewis Structures for the following molecules: Water H 2 O Ethane C 2 H 4 Nitrogen N 2 Carbon monoxide CO